Photovoltaic devices such as photovoltaic modules or cells, can include semiconductor and other materials deposited over a substrate using various deposition systems and techniques. One example is the deposition of a semiconductor material such as cadmium sulfide (CdS) or cadmium telluride (CdTe) thin films over a substrate using a VTD system. A VTD system may use a powder delivery unit, a powder vaporizer and vapor distributor, and a vacuum deposition unit.
VTD powder vaporizers are designed to vaporize or sublimate raw material powder into a gaseous form. In conventional powder vaporizers, raw material powder from a powder delivery unit is combined with a carrier gas and injected into a vaporizer formed as a permeable heated cylinder. The material is vaporized in the cylinder and the vaporized material diffuses through the permeable walls of the vaporizer into a vapor distributor. The distributor typically surrounds the vaporizer cylinder and directs collected vapors towards openings which face towards a substrate for thin film material deposition on the substrate.
FIG. 1A illustrates one example of a conventional vapor transport deposition system 20 for delivering and depositing a semiconductor material, for example CdS or CdTe onto a substrate 13, for example, a glass substrate 13 used in the manufacture of thin film solar modules. Inert carrier gas sources 25 and 27, for example, Helium gas (He) sources, respectively provide a carrier gas to powder feeders 21 and 23, which contain CdS or CdTe powder material. The gas transports the semiconductor material through injector ports 17, 19 on opposite ends of a vaporizer and distributor assembly 10. The vaporizer and distributor assembly 10 vaporizes the semiconductor material powder and distributes it for deposition onto substrate 13.
FIG. 1B is a cross-sectional view, taken along the section line 2-2 of FIG. 1A, of one example of a conventional powder vaporizer and distributor assembly 10. The vaporizer 12 is constructed as a heated tubular permeable member. It is formed of a resistive material which can be heated by the AC power source 29 and vaporizes, for example, a CdS or CdTe semiconductor material powder transported by the carrier gas into vaporizer 12 through injection ports 17, 19. The distributor 15 is a housing formed of a thermal-conductive material, for example, graphite or mullite, which is heated by radiant heat from vaporizer 12 and/or from another source. The housing of distributor 15 surrounds vaporizer 12 to capture CdS or CdTe semiconductor material vapor that diffuses through the walls of vaporizer 12. The semiconductor material vapor is directed by distributor towards a slot or series of holes 14 which face a surface of a substrate 13, which moves past the distributor 50. More detailed examples of VTD systems of the type illustrated can be found in U.S. Pat. Nos. 5,945,163, 5,945,165, 6,037,241, and 7,780,787, all assigned to First Solar, Inc.
Vaporization of powder material is often incomplete, causing clogging and deterioration of the vaporizer and distributor assembly 10. This problem may be compounded when certain depositions introduce a dopant into the semiconductor material which can react with semiconductor material and form a solid phase compound and a vapor phase compound within vaporizer 12 during the deposition process. For example, to dope a CdTe material, a process gas, such as compressed dry air, is also introduced into the vaporizer 12 to provide a reactive mix with the dopant, the latter of which is provided in the CdTe powder mix. Introduction of the dopant and process gas into vaporizer 12 can cause formation of a gas phase product and a solid phase product. While the gas can pass through the porous walls of vaporizer 12 for deposition on a substrate 13, the solid cannot and is confined within the vaporizer causing increased vaporizer pore clogging. A major hindrance to efficient and cost effective production of cadmium sulfide (CdS) or cadmium telluride (CdTe) thin films using a VTD system is the complete shut-down of production for maintenance or repair of the vaporizer and distributor assembly 10. Complete shut-downs of production for such maintenance or repair can be costly and time consuming.
An improved vapor transport deposition system which mitigates against the noted problems and which can better control the vapor applied to a substrate is desirable.